The natural intervertebral disc (discus intervertebralis or discus) forms part of a motion segment of the back. Such a motion segment consists of two vertebrae, an intervertebral disc (a cartilaginous disc which connects the vertebrae), two small posterior joints (facet joints), ligaments and muscles. All these elements jointly determine the extent to which movement of the back is possible. The intervertebral disc thus forms part of the overall locomotor apparatus, in which all the elements work together. This unit of elements can only perform its function if all the elements remain intact or are repaired in case of damage. If the intervertebral disc is for example insufficiently capable of performing its function because it is damaged, this may lead to the small joints being overloaded, which in turn may cause damage to said joints. Timely replacement of a damaged or defective intervertebral disc, preferably by an artificial intervertebral disc having comparable characteristics, is of major importance, therefore.
A natural intervertebral disc is built up of a gelatinous core (nucleus pulposus), which is enclosed in a fibrous ring (annulus fibrosus) (White et al., Clinical biomechanics of the spine, J.B. Lippencott Company, Philadelphia, 1978). The nucleus and the annulus both comprise stiff and mutually crosslinked collagen fibres, which are intertwined with proteoglycane chains. Said proteoglycane chains contain fixed, strongly negatively charged side chains (glycosamino glycanes), which interact with ions from the environment, as a result of which water is attracted by the intervertebral disc. Because of the high concentration of proteoglycane chains, the nucleus contains 85-95% water, whilst the annulus, which comprises a relatively great deal of collagen fibres and less proteoglyclane chains, contains 70-85% water. As a result of this specific composition, the disc allows movement of the vertebrae relative to each other and, in addition, has a shock-absorbing function. The two adjacent vertebrae comprise end plates consisting of hyaline (“vitreous”) cartilage, which serves as a transition zone between the soft intervertebral disc and the hard vertebrae.
In case of back complaints associated with degeneration of the intervertebral disc, such as a serious hernia nucleus pulposus, surgery may be necessary. In some cases part of the intervertebral disc—generally the annulus—can be saved, but in serious cases the intervertebral disc must be replaced in its entirety. To restore the function of the vertebral column, a prosthesis must be implanted, which prosthesis takes over the mechanical function of the natural intervertebral disc, both as regards mechanical stiffness and as regards swelling behaviour.
Prostheses for intervertebral discs must meet a number of requirements (Eijkelkamp et al., The International Journal of Artificial Organs, 2001, 21 (5), 311-321) such as, inter ails, a correct geometry for an optimum attachment and an optimum pressure distribution in relation to the adjacent vertebrae. Further requirements include a sufficient degree of stiffness so as to obtain a good shock absorption, and a swelling behaviour comparable to that of the natural intervertebral disc.
U.S. Pat. No. 5,824,093 discloses a nucleus prosthesis (10) comprising a core of a gel material (12) and a woven fibre envelope (14).
WO 2005/013863 (present inventors) discloses a prosthesis comprising a core of a gel material (2) and a wrapped fibre envelope surrounding said core (3).
US 2002/026244 discloses a nucleus prosthesis comprising a core of hydrogel with a woven fibre envelope which surrounds the core.
EP 0 353 936 discloses a prosthesis comprising a core of a gel material with an envelope of a woven material.
U.S. Pat. No. 5,192,326 relates to a prosthesis comprising a core of a gel material enclosed by an envelope of a woven material.
International application WO 04/049980 relates to an artificial intervertebral disc comprising a core of a flexible material (silicone rubber) having the shape of a flattened body, with a lower and an upper side, which upper and lower side are connected by a lateral surface, around which core substantially radially oriented windings of a traction-resistant fibre have been applied.
A drawback of such an artificial intervertebral disc is that the windings of traction resistant fibres thus applied give the fibre structure insufficient shape stability and consequently allow only a limited buildup of swelling pressure in the core without loss of the original shape, which adversely affects the durability and strength of the artificial intervertebral disc. In other words, upon application of a substantial load the core material will bulge out between the windings of the fibres.
An example of a prosthesis for replacing intervertebral discs which was and/or is commercially available comprises a prosthesis based on a flexible core and rigid end plates, viz. a rubber core covered by titanium end plates (Acroflex®). Prostheses intended for replacing only the nucleus of an intervertebral disc, which prostheses are based on hydrogel materials, are known inter alia from U.S. Pat. Nos. 5,674,295; 6,402,784 and 5,047,055, in which the natural annulus is filled with a hydrogel material, which hydrogel material has swelling properties and which may or may not be surrounded by a membrane. Other prior art prostheses are so-called hinge prostheses, such as the Propisc II®, the Maverick® and the Charity® The drawback of such hinge prostheses is that wear occurs on the hinging parts. Prosthesis comprising a flexible core do not move via hinges but through deformation of the flexible core, which does not result in wear.